A projection objective can be used in a lithography projection exposure apparatus to print a precision circuit pattern on a substrate (e.g., a wafer, a plate or the like) that is coated with a photosensitive material. The circuit pattern to be printed is typically provided by an object referred to as a reticle, which is imaged by the projection objective onto the substrate.
A projection objective of a lithography projection exposure apparatus generally includes optical elements between an object plane and an image plane. The term “between” is to be understood as meaning that the optical elements are arranged in a manner such that projection light coming from the object plane and arriving at the image plane reaches each optical element at least once. Terms like “before” and “after” are to be understood to refer to the order of the projection light propagating through the projection objective.
There are different types of projection objectives, generally referred to as dioptric, catoptric and catadioptric projection objectives. Generally, the optical elements in a dioptric projection objective are only refractive optical elements. Generally, the optical elements in a catoptric projection objective are only reflective optical elements. Generally, the optical elements in a catadioptric projection objective are refractive optical elements and reflective optical elements.
A projection objective is commonly referred to as an immersion projection objective if there is an immersion liquid between the last optical element of the projection objective and the image plane. Generally, in such an objective, the immersion liquid itself does not count as an optical element. A projection objective is commonly referred to as a double immersion projection objective if the projection objective has two immersion liquids and the last optical element of the projection objective is between the two immersion liquids. A double immersion projection objective is a type of immersion projection objective.